Conventionally, liquid crystal display devices of a monolithic type, used as a display device, are generally adapted to sample video data via analog switches that are controlled by shift registers.
For example, as shown in FIG. 11(a), the liquid crystal display device includes a pixel capacitor CLC and an auxiliary capacitor Cs that are connected to each pixel in a pixel section, and TFTs (Thin Film Transistors) are provided at the intersections of a plurality of gate lines in the row direction and a plurality of data lines in the column direction. The gate lines are connected to a gate driver and the data lines are connected to a source driver.
Further, counter electrodes are provided opposite the TFTs and a liquid crystal layer. The counter electrodes receive a counter voltage VCOM to drive the liquid crystal.
The gate driver successively outputs gate pulses to sequentially scan the gate lines and select pixels of a row per horizontal period.
The source driver, using the shift registers, sequentially scans the data lines per horizontal period, so as to point-sequentially write video data to the pixels of a sampled and selected row. This enables image display.
The source driver includes shift registers and analog switches, as shown in FIG. 11(b).
As in normal display as above, in testing the driver for malfunction, the analog switches that are controlled by the shift registers are also used to sample video data Vvideo 1 and Vvideo 2, which are supplied to the source driver.
Namely, testing of the driver is carried out in the same manner as in normal display in that the driver is first operated to write data into the pixels and operated again to read out the charge held in each pixel, using the analog switches. The operating state of the driver is tested or judged by this operation, by way of finding defect pixels.
However, such a testing method is bound to a problem in that the S/N ratio and the accuracy of testing suffer as the pixel capacitance becomes smaller and the amount of charge read out is reduced, as in a small and fine resolution display device for projection use. The S/N ratio could be improved by increasing the number of measurements. In this case, however, testing needs to be carried out for extended periods of time.
Further, the foregoing method judges malfunction of the driver according to characteristics of defect pixels. Accordingly, data processing takes time, which lowers testing efficiency.
As a countermeasure, there has been proposed a display device of a structure in which pads are used to test the driver circuits (scanning circuits), for example as shown in FIG. 10.
In this example, two driver circuits are provided to render the display device redundancy. For example, even when one of the driver circuits fails, the operations of the display device as a whole are ensured with the normal driver circuit.
In another type of display device, as shown in FIG. 12, pads 103 are provided within or on the periphery of a gate driver 101 and a source driver 102. The pads 103 are fed with the output from the last stage of the scanning circuit of the driver and a testing probe is brought into contact with the pad 103 to detect a signal. Testing accuracy and testing efficiency are improved this way.
Further, Japanese Publication for Unexamined Patent Application No. 194421/1994 (Tokukaihei 6-194421; published on Jul. 15, 1994) proposes a semiconductor device in which a number of testing circuitry that is required for function tests is installed in a plurality of target circuits. The testing circuitry includes means for storing information for the function test, and means for transmitting structural information of the target circuit to the testing circuitry. In this way, the structure of the testing circuitry can be simplified and the testing means can be provided more conveniently, thereby carrying out BIST (Built-In Self Test) more efficiently.
However, the structure shown in FIG. 12 requires a plurality of pads 103, which are specifically designated for testing. Further, the provision of the two gate driver circuits increases the number of testing pads required, as shown in FIG. 10.
This inevitably increases the pad area under severe constraints on driver area particularly in small and fine resolution panels, mobile devices, and small and fine resolution display devices for projection use. Further, in view of space restriction on wiring and protection against ESD (Electrostatic Discharge), the stylus of the probe card must accommodate to the array of pads that greatly differs from that of input terminal pads for receiving external input signals.
Further, because the large area pads are directly connected to the logic circuits, the pads 103 function as an antenna to cause damage by ESD.
Further, the structure of the foregoing publication requires complex testing circuitry for the function tests. Installing such testing circuitry in a liquid crystal display device of a driver monolithic type is not practical when considering required processing accuracy that is determined based on the required level of micro-fabrication of the display section and the area of the display section, and when considering the circuit structure of the driver. Accordingly, there is a need for a more efficient testing method.